I recently picked up a used Motif XS Rack for $450 - I used to have a Motif ES6 then a MOX 6 for my ‘top’ board, but I used them just for their sounds live and recording. I’m pretty much a newbie navigating these things.

I reset the unit to factory defaults when I got it - when playing the Multi’s only the drums play - often leaving the upper range silent.

How are you “playing” the Multis? Are you playing a controller keyboard, or are you using a DAW to sequence?

If you’re using a controller to play live, then the Parts of the Multi have to all be on the same MIDI channel. That can be done by layering them using the Utility mode setting Layer 1-4 Parts. See pages 40 and 98 of the Rack XS Owner’s Manual (http://download.yamaha.com/file/9269).